If the general population of buyers does not expect to find grand pianos on craigslist, then why would they end up on craigslist to find it? More likely, the best hope you have is to sell it to someone who found it by accident, probably not even looking for such a thing at the time. And if selling grand pianos on craigslist is sufficiently rare, then it’ll naturally not change anyone’s expectations; it was just a lucky find.
And ofc, if you the adversarial company are purchasing out these rare sales as soon as they’re up, you can trivially maintain the illusion.
That individual sale might cost you more, but if the average seller sells for less because they don’t see a real market to sell in... it’s probably still a profitable illusion.
(I responded to your other comment, but this one covers some of the same ground.)
It still seems strange to me that this would work, when there are so many easy ways for piano enthusiasts to gather and communicate, and when it's such an expensive item (so if you knew the market was being manipulated, you wouldn't just throw up your hands and accept the hit, you'd look for a better deal).
I might be getting confused about the basic claim being made, though -- if sellers can achieve decent prices when they choose to demand them, but often don't realise this and therefore get ripped off by professional buyers; and if buyers can save money by buying directly from individual sellers, but are often unaware of this option (because there aren't a whole lot of pianos being sold this way, and they're not on the market for long); then I guess that does make sense.
And ofc, if you the adversarial company are purchasing out these rare sales as soon as they’re up, you can trivially maintain the illusion.
That individual sale might cost you more, but if the average seller sells for less because they don’t see a real market to sell in... it’s probably still a profitable illusion.